1997 5.7 Vortec Stumbles

I'm working on a 97 Silverado 5.7 Vortec that runs great for the first 10 min or so, then stumbles while in overdrive. I've replaced the rotor & cap, tested fuel-pressure, plugs & wires, throttle position sensor, new intake manifold gaskets, has a new transmission, can't find any vacuum leaks, not throwing codes, Sea-foamed it, even tried premium gas.It's progressed to an occasional stumble while in idle. Short of giving in and turning it into the shop, I'm out of ideas.

positive battery post to one of the large fuses in the underhood fuse box.. 0.04 volts.. why.. because i have found the cable that supplies power to this box loose.. corroded at the hidden connection...

the above will take you about 3 minutes to perform.. less time than it took me to type it..

and in front of the drivers side rear tire.. look at the top of the frame rail.. check the ground connection for the fuel pumps...

that's a KNOWN failure there..

why... the voltage drop tests.. i have several friends with 95 to 98 chevy pick ups.. and they have all had ground issues.. i measured one on test 4 between the engine and the body at 8.5 volts.. difference between the engine and the body..

with the engine running.. figure out which relay in the fuse box is the fuel pump relay... while the engine is running.. pull the fuel pump relay.. the truck should continue to run.. if it dies.. then change the oil pressure switch..

examine the pins on the bottom of the fuel pump relay.. also pay attention to how hard it is to insert into the socket.. i have had to use a sharp tipped tool to get behind the tongue of the terminal in the socket.. tightening up the grip of the terminals on the pins on the relay..

please also examine the sides of the ignition coil for signs of high voltage leaks..

do you have a friend with an advanced scan tool.. that can get you into the OEM tests.. that allow you to command the EGR valve open and closed...

get into those tests.. command it open to 90 percent while you keep the engine running with your right foot.. its going to try to die.. it takes a lot of throttle...

close the egr.. open it again.. close it..

then .. at engine off.. operate it to 100% and closed a bunch of times..

start it again.. see if it idles smooth..

have somebody take the truck out for a spin.. monitor the EGR position with the scan tool.. you should see around 40% under acceleration. and 30 to 35% at cruise..

egr valves get clogged.. when they fail to open.. or to flow when they open..

the fuel injection system will be TOO LEAN and it will ping and misfire..

and.. does your truck have over 150K miles on it.. with a test light.. start probing the various fuses that come on with the key.. this takes 2 people... one in the drivers seat.. one on the test light.. if the test light flickers while you are moving the key.. you could have dirty contacts in the ignition switch..

First of all, thanks for the information. I measured the ohms & volts from the negative to the block: all good. I pulled the fuse & relay mount to check the condition of the cable connections: good. I pulled the fuel-pump relay after it was started to check the oil-pressure sw: still ran.I did see a little sign of burning on the ignition coil so I've ordered one & will let you know how it's installation affects the stumbling.I pulled the distributor about a month ago when changing the intake manifold gaskets; gear teeth are still good without much lash and there is no side-to-side wobble. I don't have the advanced diagnostic tools to really check the EGR valve so I replaced it: truck idles a little better but the stumble is still there. Fuel pressure is 56psi and immediately jumps over 60psi when I bump the throttle. Until I get the new ignition coil installed, I'm running a fuel pressure gauge and will monitor the pressure when it starts to stumble again. Maybe I'll get lucky.

do you have a friend with an advanced scan tool.. that can get you into the OEM tests.. that allow you to command the EGR valve open and closed...

get into those tests.. command it open to 90 percent while you keep the engine running with your right foot.. its going to try to die.. it takes a lot of throttle...

close the egr.. open it again.. close it..

then .. at engine off.. operate it to 100% and closed a bunch of times..

start it again.. see if it idles smooth..

have somebody take the truck out for a spin.. monitor the EGR position with the scan tool.. you should see around 40% under acceleration. and 30 to 35% at cruise..

egr valves get clogged.. when they fail to open.. or to flow when they open..

the fuel injection system will be TOO LEAN and it will ping and misfire..

The timing light is useless on this vortec truck. The only thing that can cause p1345 cam crank correlation is a improperly installed distributor or incorrect cam retard #.

Once you put the distributor in you must look at cam retard on a scanner the spec must be 0 + or- 2 anything else will set p1345. You set the cam retard number by turning the distributor and looking at a scanner instead of a timing light.